Back in SharePoint 2010 Microsoft introduced the capability to display an exchange calendar over the top of a SharePoint calendar.
The exchange calendar has always had it’s limitations. But here is another I recently discovered.
If you are looking to configure a SharePoint online calendar to consume an exchange online calendar as an overlay, the short answer is you can’t do it! Why?
It was not an easy explanation from Microsoft to find. But see below a snippet and link to a support article from Microsoft.
“This issue occurs because the ability to use the calendar overlay in a SharePoint Online calendar that is retrieved from Exchange Online isn’t supported.
If you were to try to complete the calendar overlay following microsoft’s support guide you would receive an error
“The HTTP request is unauthorized with client authentication scheme ‘Ntlm’. The authentication header received from the server was ‘Basic Realm=””
It is a little disappointing that this cannot be completed, perhaps the introduction of Office 365 Planner & Groups will see this change in the near future.